The present invention relates to communication antennas and to RF signal transmission through a dielectric barrier. More particularly, it relates to a new and improved glass mount mobile vehicle antenna system employing very high Q, high dielectric constant, low loss dielectric resonators, together with an elevated feed antenna to couple RF energy through the glass via resonance mode coupling of the resonators to minimize coupling losses and to provide an improved omni-directional communication antenna system having high radiation efficiency and low pattern distortion.
Technological advances in personal communication services and products have been astounding. The development of personal paging/beeper systems and mobile cellular telephone systems are prominent examples of these developments. An ultimate technological goal in this field envisions individuals carrying small, inexpensive hand-held communicators and being reachable by voice or data with a single phone number, no matter where they are. This new system, generally referred to as a Personal Communications Network (PCN)/Personal Communications System (PCS), is a wire- less, "go anywhere" communicator system which eliminates the need for separate numbers for the office, home, pager, facsimile or car. Many national and international bodies responsible for regulating communications networks and for working out international communication standards have generally set aside a portion of the ultra-high frequency microwave radio spectrum within the band from about 1.5 GHz to about 2.4 GHz as the bandwidth range dedicated for PCN/PCS communication systems. The present invention is directed to mobile antennas and especially window mounted mobile vehicle antennas for use in any communications system, but which are especially adapted for use in the high frequency operating ranges intended for PCN/PCS communications.
Glass mount mobile antennas for use in cellular mobile telephones, for example, are known which mount on the window of the vehicle, thereby avoiding the need to drill holes in or otherwise modify the vehicle body. Window mounted antennas include an outside module on the outside of the window glass on which a generally vertical radiating element is mounted and an inside module inside the glass disposed in registration with the outside module which contains an impedance matching circuit and in some instances, a ground plane, as necessary, for operation of the antenna. Consumers have welcomed the through-glass mounted antennas because it is no longer necessary to drill a hole through the vehicle which detracts from the vehicle's value. However, the blocking effect of the passenger compartment, coupled with through glass signal losses occurring with most glass mount antennas, provides an antenna having a lower gain and a higher pattern distortion than the roof-mounted antenna. Gain, for example, is normally in the 1-3 dB range. Most cellular telephone communications occur at operating frequencies of about 800 MHz. Even at these lower frequencies, improved coupling efficiency and lower distortion is desired.
Efforts at improving the performance of prior art glass mount mobile antennas have employed capacitive couplings through the vehicle glass and low-Q circuits involving LC impedance matching networks. For example, in U.S. Pat. No. 4,089,817 to Kirkendahl, a capacitively coupled antenna system is described. The capacitive coupling consists of electrical patches on both sides of the automobile glass, such as a windshield or window, which forms a capacitor to couple the RF energy. In U.S. Pat. No. 4,839,660 to Hadzoglou, an improved structure including a moderate coupling impedance wherein the bottom radiation element is close to a complete half-wave dipole is described. A full-dipole radiation element cannot be used because of the high transmission impedance sensitivity at one half wavelengths. Other illustrative examples of glass mount antennas employing different circuits to provide impedance matching networks for capacitive couplings include U.S. Pat. No. 4,992,800 to Parfitt; U.S. Pat. No. 4,857,939 to Shimazaki; and U.S. Pat. No. 4,785,305 to Shyu.
Each of these previous efforts to provide capacitive coupling by positioning electrical patches on both sides of the vehicle glass presents a number of attendant disadvantages. The electrically conductive patches generally may not be made large enough in comparison with the operating wavelength to keep it from being the primary radiating element. Accordingly, only high impedance couplings on the order of several hundred Ohms may only be provided which leads to high losses due to leakage of the electrical field at higher frequencies. At higher frequency bands like the proposed PCN/PCS band, even a small conductive patch is no longer effective to act as a lump capacitor element considering the thickness of the vehicle glass. A capacitance PI circuit bypasses the signal and makes it more difficult to match the high impedance of the antenna to a 50 Ohm system. In U.S. Pat. No. 4,764,773 to Larsen, an improved coupling structure is proposed, including two patches to reduce the coupling impedance. The Larsen antenna, however, still suffers from having the capacitor coupling limitation of requiring small patch sizes. At higher operating frequencies, this problem still exists or is exacerbated.
For mobile communications, it is critical to provide an antenna system having low pattern distortion. A whip collinear antenna does not always have a uniform current distribution. Frequently, a lower section has the strongest radiation. In a real-life automobile or other vehicle situation, the lower section of these antennas is actually blocked by the roof of the vehicle, causing severe pattern distortion and deep null. This situation is made worse at the higher proposed frequencies for PCN/PCS because the length of the radiators are only half that of the cellular band radiator, due to the doubling of the operating frequencies. A collinear array having a high feeding point, is normally provided by applying a de-coupling sleeve or by means of slot technology. These antennas normally have a 50-75 Ohm impedance which makes it difficult to adapt these antennas to the capacitively coupled prior art structures. As a result, outside impedance matching networks must be used to achieve a 50-50 Ohm transmission and even higher losses are expected at-the higher PCN/PCS frequency bands when these at conventional LC circuits are employed.
Another major shortcoming of prior art capacitively coupled antenna systems is that these kinds of systems suffer strong spurious emission to the passenger compartment simply because the whip collinear array needs a ground plane. Prior art methods used to isolate the feeding line from environmental radiation have relied on the couplers themselves to act as an impedance matching network. For example, in the above-mentioned Hadzoglou and Kirkendall patents, the coupling patches are part of the antenna's impedance matching network. In U.S. Reissue Patent 33,743 to Blaese, another capacitively coupled antenna system is described which attempts to couple the coaxial cable through the glass. At high frequency PCN/PCS bands the proposed 1/4 wave antenna would be only about 1.7 inches long, which would lie completely below the roofline of the vehicle causing severe pattern distortion and deep null. In U.S. Pat. No. 4,939,484 to Harada, a coupler including helix cavities is used to couple the signal through the glass. Unfortunately, the aperture is fixed to satisfy the 1/3 object frequency, as described in the Harada patent. For an 800 MHz cellular application, the helix should be designed for a 200 MHz frequency which has a Q factor of over 1,000 and enough coupling aperture. However, at a 1.8 GHz frequency band, the helix must be designed for 600 MHz. A 600 MHz helix cavity will have a small aperture of only about half that of the cellular band. A significant drop of unloaded Q is unavoidable, due to the thin helix and the coupling co-efficient is not sufficient to keep a 10% band width. Other drawbacks of the helix cavity approach described in the Harada patent are that in the proposed antennas, it is difficult to tune the frequency of the antenna system and it is difficult to manufacture the antennas because of its complicated structure.
Generally, the performance of the prior art antenna systems degrades considerably as frequencies approach the 1.5 GHz to 2.4 GHz range proposed for PCN/PCS communications. The prior art antennas and systems are relatively low frequency systems, when compared to microwave frequencies and they all employ low Q, lumped LC elements, or semi-lumped elements provided by placing the LC elements in metal enclosures. At the higher PCN/PCS frequencies, the losses of LC circuits will increase considerably due to the low, unloaded Q nature of the prior art systems and components. The PCN/PCS communication systems must operate at low power levels of about 1 Watt and provide a very wide range of coverage at very high frequencies. The prior art antenna systems are inappropriate for satisfying these requirements because of their low frequency approaches.
It is known in the filter arts that certain dielectric resonators may be used to build high-quality, narrow-banded filters, typically less than 2%. In filter applications, the dielectric resonators are normally placed in a continuously conductive enclosure to minimize any losses which may arise due to spurious modes or leakage. Illustrative dielectric resonators are described in U.S. Pat. No. 2,890,422 to Schlicke. The dielectric resonators have very good long-term stability so that component aging effects are negligible. The high density nature of the resonators reduces the undesirable effects of moisture to a minimum. Even at high frequency bands around 1.8 GHz, dielectric resonators may still maintain an unloaded Q factor of greater than about 3,000. In contradistinction, the helix cavities with a 600 MHz based frequency described in the Harada patent, cannot achieve such a high Q factor. The hollow-cavity helix systems described in the patent are more sensitive to the environment than dielectric resonators and special sealing is required to keep the Q from dropping further. Furthermore, it is impossible to keep sufficient coupling coefficient for a small helix aperture through a vehicle glass having a thickness from about 4 to about 6 mm, plus the thickness added by any adhesive mounting pads. The dielectric couplers solve the aperture problem of the Harada patent because the dielectric constant can be selected. For example, at 1.8 GHz, a dielectric resonator with a dielectric constant of 80 available commercially from Trans-Tech, Inc. under the trade designation 8600 Series has a dimension of D=24 mm, and h=7.6 mm. At 2.4 GHz, a dielectric resonator with constant of 38 also commercially available from Trans-Tech, Inc. under the designation Series 8800 may be used which has the dimensions D=24 mm and h=9.6 mm, which still provides a large enough aperture to maintain the coupling coefficient at a desirable level. On the other hand, an 800 MHz base frequency helix may have only a 10 mm aperture.
Accordingly, to overcome the shortcomings and disadvantages of the prior art systems and devices, it is an object of the present invention to provide a new and improved glass mount antenna system.
It is another object of the present invention to provide a glass-mount antenna system adapted to operate at upper UHF and higher microwave frequencies exhibiting greater coupling efficiency and less pattern distortion than has heretofore been achieved.
It is a further object of the present invention to provide a coupling scheme including a new and improved tuneable wide band coupling structure which provides flexible impedance matching to permit the feeding point of the antenna to be raised easily.
It is still another object of the invention to provide an antenna system having improved emission performance by employing twin enclosed cavities containing moisture-insensitive, high Q dielectric resonators and by implementing a feeding line isolating choke at the antenna end.
It is still a further object of the present invention to provide a glass-mount antenna system employing a resonance mode coupling, such as TE011 and TE111 modes, instead of electrical capacitance or inductance couplings.
It is still another object of the present invention to provide a high performance omni-directional PCN/PCS communication antenna system capable of coupling high frequency RF energy through a dielectric wall without the need for a continuously conductive enclosure and without significant losses.